The field of the present invention relates to free-space optical power transfer. In particular, optical assemblies for free-space optical propagation between waveguide(s) and/or fiber(s), and fabrication methods therefor, are disclosed herein.
Many optical components cannot yet be implemented within a waveguide or optical fiber, but require so-called “free-space” propagation of optical power through the component. The transverse dimensions of such components are typically too large to provide transverse confinement or guiding of the propagating optical power, which will converge and/or diverge as it propagates through the component. When such components must be incorporated into an optical transmission system that includes one or more planar-waveguide(s) and/or optical fiber(s), additional focusing and/or collection optics are required for: 1) transforming a small guided mode emerging from an end of an optical fiber or planar waveguide (typically less than about 10 μm across and divergent upon leaving the fiber or waveguide) into a free-space optical mode that may be transmitted through the optical component; and/or 2) collecting the free-space optical mode and transforming it into an optical mode (typically convergent) that may be efficiently coupled into another optical fiber or planar waveguide. The overall efficiency of optical power transfer between the fiber(s)/waveguide(s) is determined to a major extent by the degree of spatial mode matching achieved between the fiber/waveguide optical modes by the additional focusing and/or collection optics.
Exemplary prior art dual-lens optical assemblies are shown in FIGS. 1 and 2, where an optical isolator 40 (comprising in this example a Faraday rotator with input and output polarizers cemented onto the faces thereof) is shown positioned between two lenses 22 and 72 (ball lenses in FIG. 1, spaced from the fiber ends as shown or alternatively in contact with the fiber ends; gradient-index [GRIN] optical fiber coupling segments fusion spliced onto the fiber ends in FIG. 2). The optical modes are approximately indicated by the dashed lines in FIGS. 1 and 2. Optical power propagating through a single-mode optical fiber 20 exits the fiber end and is then focused by lens 22 for propagation through isolator 40 (with decreased divergence, substantially collimated, or convergent). Once through the isolator 40, the propagating optical power (typically, but not necessarily, divergent at this point) is collected and coupled into single-mode fiber 70 by lens 72. Optical transmission between fiber 20 and fiber 70 through isolator 40 is kept above operationally acceptable levels (i.e., the lenses provide adequate spatial mode matching between the two fibers) only within tight longitudinal, transverse, and angular alignment tolerances for both fiber ends and lenses (typically a few μm or less). Achieving alignment within these tolerances typically requires expensive and time-consuming active alignment procedures, driving up costs for assembled devices (“active alignment” denoting a procedure in which optical power transmission through the fibers/lenses is monitored for guiding the alignment procedure; in contrast, a “passive alignment” procedure does not require optical power transmission during the alignment procedure). Furthermore, while the solutions shown in FIGS. 1 and 2 may be adequate for some in-line fiber-optic applications, there is also a need for solutions compatible with semiconductor-based active optical devices, such as lasers and modulators, and/or compatible with planar waveguide optical transmission components. Optical mode sizes in these cases may be smaller (sometimes less than 1-2 μm across) and divergences correspondingly larger, imposing even tighter alignment tolerances for achieving an operationally acceptable level of optical power transfer.
Various exemplary embodiments of single- and dual-lens optical assemblies and methods for constructing the same are disclosed herein which may overcome one or more of the drawbacks of the previous art (as described hereinabove).